1. Field of the Invention
This invention relates to a method for removing arsenic sulfide mineral impurities from talc and is especially directed to a wet process for removing small quantities of finely divided arsenic sulfide minerals from finely divided talc to produce high purity products.
Talc is a naturally occurring nonmetallic mineral which in theoretically pure form has the formula H.sub.2 Mg.sub.3 (SiO.sub.3).sub.4, corresponding to 3MgO.4SiO.sub.2.H.sub.2 O. The mineral occurs in ores which usually contain varying quantities of accessory nonmetallic minerals such as tremolite, serpentine, magnesite, calcite and quartz. Some deposits also contain metallic impurities such as iron, nickel, bismuth and arsenic in the form of sulfides, frequently mixed sulfides. One of the largest domestic talc industries, located in Vermont, processes an ore in which the principal minerals are talc of the platy type and magnesite which occurs in the form of breunnerite (a magnesium iron carbonate). The accessory minerals are reported to include pyrite and pyrrhotite (iron sulfide minerals) as well as gersdorffite (NiS.sub.2.AsS.sub.2) and cobalt minerals.
For some important uses of talc, ore of suitable composition is merely ground and sized. Accessory minerals including metallic impurities are not removed. Talc used in other products such as paints must meet stringent specifications for purity, color, carbon dioxide content, oil absorption and fineness of grind. The production of such products may necessitate beneficiation of an ore to remove nonmetallic minerals. Carbonate minerals such as the magnesite in the Vermont ores is removed from the ore along with the bulk of accessory sulfide minerals by froth flotation. Especially stringent limitations are imposed by the Cosmetic, Toiletry and Fragrance Association (CTFA) for talc used in cosmetics. CTFA F1-1, Parts I-A and II specifies a maximum of 2 p.p.m. acid-soluble arsenic.
A limited amount of the talc that is processed into powdered products meets the standards established for the cosmetic industry. Domestic talc meeting the standards is in short supply. A factor in the shortage is that some available processed powdered talc products contain residual arsenic impurities in quantities which are small but are in excess of the maximum tolerable level. The arsenic, present as a complex sulfide, cannot be removed to a sufficient extent by conventional hydrometallurgical techniques or by gravity separation. Even flotation beneficiated talc products may contain too much arsenic to be acceptable by the cosmetic industry. Simple leaching of talc with various mineral acids, caustic solutions or even water may remove a limited amount of arsenic. However, only a portion of the arsenic is removed.
2. Prior Art
The use of hot nitric acid to solubilize metal sulfides which are insoluble or poorly soluble in common solvents is well known in analytical chemistry. A redox reaction is involved and the sulfide ion is reduced to elemental sulfur. Such a reaction has been used to decompose gersdorffite, forming arsenic acid and elemental sulfur. (J. J. Mellor, INORGANIC AND THEORETICAL CHEMISTRY, Vol. IX, page 310, 1947 Edition.) Mixtures of nitric and sulfuric acids are used by the metallurgical industry to dissolve a variety of metal sulfides or mixtures of sulfides. This combination of reagents has been recommended in a two-stage treatment for separating arsenical minerals from nickel and cobalt. Arsenic is precipitated as the crystallized oxide which is separated from solubilized nickel and/or cobalt. The process is described in French Pat. No. 1,125,038 to Borvali et al.
A well-known method for removing metallic impurities such as arsenic sulfides from silicate ores and minerals involves forming volatile metal chlorides. U.S. Pat. No. 2,504,357 to Swallen discloses such a process.